Haptic actuators are commonly used with electronic devices to provide the user with a sensory signal also referred to as haptic feedback. For example, mobile phones are often equipped with a haptic actuator which vibrates to provide a notification for a user, for example as an alert that a text message has been received.
To this end different types of haptic actuators have been developed, among which are the eccentric rotating mass, ERM, and the linear resonant actuator, LRA.
ERMs are based on a single-phase DC motor driving an eccentric mass. The system being asymmetric, spinning the mass creates a force proportional to the velocity of the mass squared divided by the radius of rotation, that is then perceived by the user. Upon rotation of the mass, a back electromotive force, BEMF, is generated across the motor that opposes the voltage of the source that created it. The BEMF is proportional to the frequency of rotation of the mass and can be used to provide a feedback to a controller operating the motor.
ERMs have a relatively slow startup time and a low efficiency in converting electrical to mechanical energy. In addition, the vibrational strength of ERMs depends on the frequency of oscillation. These properties limit the use of ERMS for haptic applications.
LRAs are based on an inductive coil coupled to a spring holding a permanent magnet. In operation, the spring and mass system move along a single axis. When a current is passing in one direction through the coil it creates a magnetic field that repels the magnet. When passing the current in the other direction the magnetic field attracts the magnet. The system has a mechanical resonance frequency typically in the range of 50-300 Hz. The resonant frequency provides the optimal push/pull combination in time to drive the magnet at its maximum acceleration. In addition, the system has a relatively high Q factor, which means that when driven off resonance it produces little motion. The BEMF of the LRA is proportional to the amplitude of its oscillations.
Compared with ERMs, LRAs are approximately twice more efficient in converting electrical to mechanical energy. Additionally, LRAs provide a well-controlled haptic feedback to the user, as only the amplitude of vibration of the system varies and not its frequency.
The resonant frequency of LRAs varies due to manufacturing process. As a result, the resonant frequency may differ from the specified value, with an error of about 10%. Additionally, the resonant frequency depends on the mass to which the motor is attached, temperature, lifetime degradation, and the amplitude of the motor vibration (resonant point changes based on AC signal amplitude). All these variations mean that driving the LRA with a fixed frequency is not sufficient to achieve the maximum system performance. Additionally, even if the resonant frequency of a specific motor is known before applying signal to it, the signal itself could change the resonance point. These factors mean that driving an LRA motor at its most efficient and highest-vibrational strength point requires a closed loop system that actively tracks the resonant frequency.
Different systems have been designed to control ERMs and LRAs. In these systems, the haptic actuator is being driven by a voltage signal and the BEMF is sensed and used as a feedback signal. In a first approach, the BEMF can be sensed only once the driving of the haptic actuator has been interrupted. In a second approach, sensing channels, are used to sense on the fly the current and voltage across the LRA. This allows monitoring the frequency of operation of the haptic actuator at any time but requires a complex analog design and significant digital processing, which increases both the footprint and the power consumption of the system.